1. Technical Field
The present invention relates to registration systems and, in particular, to a system and method for providing accurate registration between a printed image and previously printed images on a moving substrate.
2. Discussion of the Prior Art
Systems for controlling the relative positions of a moving substrate and an element or elements which operate on the moving substrate have been in use for some time. Control systems of this type typically use marks or indicia which are printed on the substrate material at regular intervals. These marks are scanned by a sensor as the substrate is fed past the operating element. When a mark passes the sensor in such a manner as to produce a signal indicative of an error in registration between the mark and the operating element, the sensor, in conjunction with control circuitry, generates a correction signal and an adjustment is made in the registration.
As an early example of a control system of this type, U.S. Pat. No. 2,250,209 issued July 22, 1941 to Shoults et al., discloses a control system for a stamp perforating machine. In the Shoults system, a perforating element operates on a continuous length of moving postage stamps. The Shoults control system includes a photocell which monitors marks formed on the length of stamps and produces a corresponding signal. A second photocell produces a second signal representative of the position of the perforating element. A motor which is responsive to variations in the two signals corrects the relative positions of the stamps and the perforating element.
In a later example, U.S. Pat. No. 3,781,490, issued Dec. 25, 1973 to Phillips, discloses a reel-to-reel magnetic tape transport system. The tape carries a number of laterally spaced data track groups and prerecorded reference tracks. A control transducer senses the reference tracks and provides an output signal indicative of increments of tape movement. This information is used to derive an output signal indicative of tape speed, i.e., displacement per unit time. The control transducer also provides a signal indicative of the lateral position of the tape. The control transducer includes a tension transducer which provides an output signal indicative of the tape tension. The lateral tape position signal controls the lateral position of a data processing head. The tape speed signal and tape tension signal jointly control two reel drive motors to maintain a desired tape speed, i.e., a desired incremental tape distance per unit time, and a desired tape tension.
More recently, U.S. Pat. No. 4,569,584, issued Feb. 11, 1986 to St. John et al., discloses a color electrographic recording device which produces a composite color image on a recording medium. The St. John printer transports the recording medium along a predetermined path. A print station in the transport path of the medium includes a recording head with an electrode which forms an electrostatic latent image on the medium. A number of developing stations in the transport path develop the latent image into a corresponding visible component image of a respective color.
The registration system disclosed in the St. John patent utilizes a series of solid, spaced-apart tracking marks which are printed on the print medium adjacent both of its edges. The tracking marks are printed to have a known, constant number of print lines between adjacent marks; the constant number of print lines is representative of a given constant value. The tracking marks are observed electro-optically as the print medium moves through the device. The signals generated by the photosensors in conjunction with appropriate electronics are used to determine whether the value obtained from the photosensor observation, relating to the spacing between adjacent tracking marks, is the same or different from the given constant value. Any differences between the observed value and the given constant value are processed to form an error sample representative of the differences. A number of these error samples are then averaged to produce an error correction signal which corresponds to an average of the physical longitudinal shrinkage or expansion of the print medium that has occurred between the time the device printed the timing marks and the later time that the marks are observed. The error correction signal is utilized to prevent image misalignment.
A deficiency of the St. John registration technique lies in the fact that the actual correction to registration is not applied at the precise point on the medium from which the correction signal was generated. Rather, because the correction signal is based on an average of signals taken over a length of the medium, the registration correction is applied at a point on the medium removed from the physical source of the correction signal.
According to the St. John et al. teaching, an opto-mechanical encoder provides a series of pulses where each pulse represents an incremental distance of print medium movement. Control circuitry is provided to count the number of encoder pulses generated. To discern dimensional changes in the longitudinal direction, the control circuitry counts the number of encoder pulses occurring between adjacent printed tracking marks. As the pulse sensor observes movement of the print medium from one tracking mark to the next, the number of pulses observed between the two tracking marks will be indicative of either no dimensional change, a shrinkage of the print medium, or a stretch or expansion of the print medium. Since, as discussed above, there is a given constant value of encoder pulses associated with the longitudinal distance between tracking marks when no dimensional change has occurred in the longitudinal direction, if the print medium has stretched, there will be an increase in the number of observed encoder pulses above the constant value between the tracking marks. Conversely, if the print medium has shrunk in the longitudinal direction, there will be a decrease in the number of observed encoder pulses below the constant value between the tracking marks. These pulse counts above and below the constant value are termed samples.
To remove any noise associated with a sample signal, a number of samples are averaged and a registration correction is made based on the resulting average. This is accomplished mathematically by taking a running average over the sample group, i.e., the most current sample is added to the sample group and the oldest sample in the sample group is dropped out. Thus, the St. John device produces a running average registration correction signal utilizing measurements taken over a series of consecutive tracking marks. Therefore, the correction in registration provided by the St. John device is always "running behind" the physical source of the correction signal by the number of tracking mark intervals used to generate the correction signal.